1. Field of the Invention
The present invention relates to a boost type switching regulator for supplying a constant current so as to drive a light emitting diode (referred to as “LED” hereinafter).
2. Description of the Related Art
In various sorts of portable devices employing LEDs as light sources, such as portable telephones, currents to be supplied to the LEDs are required to be controlled at constant values in order to make light emitting amounts, luminance, or chromaticity of the LEDs constant. As power supplies for controlling LED drive currents so as to supply stable and constant currents to loads as described above, boost type switching regulators are utilized.
Switching regulator power supply apparatuses required for driving LEDs produce constant currents which are required in order to produce light emitting amounts, luminance, or chromaticity of the LEDs necessary for users. In most portable devices, the constant currents are fixed. Even when those currents are to be changed, a current required in a fine adjustment is merely changed. Generally, as an example, in a case of switching regulator power supply apparatuses for driving an LED, which are used as LCD panel display back lights of portable telephones and the like, LCD drive currents have been set to such currents capable of obtaining sufficient light emitting amounts required for display operations, and those currents have been set to fixed values.
In the switching regulator power supply apparatus, a constant current for driving the LED is determined by a detecting circuit 5 of FIG. 2. In a normal case, the detecting circuit 5 is constituted by a resistive element and the constant current is set by changing a resistance value of the resistive element. In order to set the light emitting amount of the LED constant as described above, an impedance (resistance value) of the detecting circuit 5 may be connected so as to become a predetermined impedance.
Also, a drive current ILED of the switching regulator power supply apparatus and a resistance value RLED of the detecting circuit 5 are expressed by the following equation.ILED=VREF/RLED 
In this case, ILED is a current which is supplied to a load 6, and corresponds to a constant current for driving the LED. VREF is an output voltage value of a reference voltage circuit provided in an SWR control circuit 4. Further, resistance value RLED is an impedance (resistance value) of the detecting circuit 5. The switching regulator power supply apparatus is operated in such a manner that a voltage difference generated by the current (=ILED) flowing through the impedance of the detecting circuit 5 becomes equal to the reference voltage value VREF of the reference voltage circuit provided in the SWR control circuit 4. In other words, a load current is stabilized to become the above-mentioned current ILED by turning on/off a switching transistor by a drive signal whose duty ratio is determined by a feedback signal in response to the load current.
FIG. 3 is a block diagram for showing an SWR control circuit 4 of a conventional switching regulator. The SWR control circuit 4 includes an input terminal 11, a reference voltage circuit 19, an error amplifying circuit 13, a triangular wave oscillating circuit 14, a comparator 15, and a buffer circuit 16. The input terminal 11 is used for inputting a feedback signal from a detecting circuit 5. The reference voltage circuit 19 outputs a reference voltage. The error amplifying circuit 13 amplifies a voltage difference between the reference voltage and the feedback signal. The triangular wave oscillating circuit 14 outputs a triangular wave signal. The comparator 15 compares the triangular wave signal with a signal outputted from the error amplifying circuit 13 to generate an output signal. The buffer circuit 16 converts the output signal of the comparator 15 into a drive signal, which makes it possible to drive a switching transistor Q1 provided in a power converting circuit 3.
The SWR control circuit 4 controls the switching transistor Q1 in such a manner that the feedback signal becomes equal to the reference voltage. The output of the reference voltage circuit is at a predetermined constant voltage level, so a current flowing through the load 6 is controlled to become a constant current based upon a voltage of the detecting circuit 5 and the reference voltage circuit 19 provided in the SWR control circuit 4.
Recently, since there are increasing portable devices which require a plurality of LEDs, the detecting circuit 5 or other such technique which involves connection to a single impedance is not sufficient for setting the light emitting amount of the LED constant, so a detecting circuit having a plurality of impedances corresponding to a plurality of light emitting amount outputs is necessary.
FIG. 4 is a block diagram for showing a conventional boost type switching regulator power supply apparatus for driving an LED, which corresponds to a plurality of light emitting amount outputs (refer to, for example, JP-A-2004-194448). A detecting circuit 7 corresponding to the plurality of light emitting amount outputs includes, when two levels of outputs are required, for example, a first resistive element and a second resistive element both of which are connected in parallel to each other, and a switching transistor. The switching transistor controls with a control signal which is inputted from a terminal 8 for switching output levels. Similarly, when two levels or higher levels of outputs are required, the detecting circuit 7 can be additionally provided with the resistive elements and the switching transistors if necessary.
However, in a case where a switching regulator power supply apparatus for driving an LED, which corresponds to the light emitting amount outputs of a plurality of levels is to be constituted, the resistive elements required for setting the light emitting amounts and the switching transistors controlled by the switching control signals are required. As a result, a total number of elements required for the power supply apparatus is increased, accompanied by an increase of a mounting area thereof, which impedes a reduction of costs and an area saving effect (namely, high density mounting effect).